Project Summary The gut microbiota has a fundamental role in the development and stability of the host immune system. Colonization by certain bacteria, and even individual species, can alter the course of many infectious and inflammatory diseases. Our understanding of the immune consequences of colonization by members of the gut microbiota is based primarily on laboratory mouse models. Although this conventional approach has enabled detailed mechanistic studies on the immune system, laboratory mice may not reflect the more complex diseases of humans and free-living mammals in a natural environment. Here we present the use of a unique outdoor facility that allows us to adapt established mouse models to study disease risk in a more natural environment. Our preliminary data suggests that introduction of laboratory C57BL/6 mice to our facility increases the presence and function of key immune cell populations coincident with microbiome changes. The increase in circulating neutrophils and CD4+/CD8+ memory cells, reduction in naive T cells, and increased expression of costimulatory molecules on antigen presenting cells, all occurs in the absence of viral, bacterial or parasitic pathogen exposure. Detailed mechanistic study of how a natural microbiota alters mucosal immunity and contributes to disease risk has yet to be investigated. We therefore propose that gut wild microbiota enhances local innate immune activation and promotes the host immune response towards a proinflammatory state. To test this hypothesis, we propose three aims: 1) define the effect of the wild microbiota on T cell polarization in the gut, 2) determine the role of the inflammasome in the enhanced immunity mediated by the wild microbiota, and 3) determine whether Nod2?/? mice display immune dysregulation when exposed to wild microbiome. Altogether, these studies would elucidate novel host-microbe interactions and may provide innovative insight into how genetic susceptibility can contribute to the development and maintenance of disease.